MM6, 7, 8, 9 NPC's

[Queen Hera]

If I hire an Instructor & Teacher at the same time do I get experience points from both i.e. 15% from Instructor AND 10% from Teacher? (I've played all games countless times but have only hired 1 or the another before)

[Avonu]

Yes.
1 Instructor and 1 Teacher give both their bonus.
Two Instructors or 2 Teachers give bonus only from 1 NPC.
But for me it is better to hire Instructor and Scholar - 5% less exp bonus but you don't need to worry about ID items.

[GreatEmerald]

Yeap, NPCs work exactly like equipped non-Relic items.

[Queen Hera]

Thanku

[Macros the Black]

Yes.
1 Instructor and 1 Teacher give both their bonus.
Two Instructors or 2 Teachers give bonus only from 1 NPC.
But for me it is better to hire Instructor and Scholar - 5% less exp bonus but you don't need to worry about ID items.

Indeed. The 5% exp won't make as much of a difference as the skill points you'll have left to spend on other skills by not GMing ID item.

In MM6 it may be worthwhile to take a banker and scholar at the start of the game, then switch to an instructor and scholar when you have enough gold (basically when you've bought every spell you really need). In MM7 you usually don't need more gold, you can't buy the expensive stuff until later on anyways.. And in MM8 you can't hire NPC's